doc: add documentation for request.path

The field has been added in v0.4.0.

PR-URL: https://github.com/nodejs/node/pull/25788
Refs: https://github.com/nodejs/node/commit/b09c5889bee8388a6710ecf75d76ca242e0684bd#diff-1c0f1c434b17b7f8795d44a51a14320aR814
Refs: https://github.com/nodejs/node/blob/380929ec0c4c4004b522bed5e3800ebce2b68bfd/test/parallel/test-http-client-defaults.js


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>